Khan, 70, has been at the heart of political turmoil since he was ousted as prime minister in a vote of no confidence last year, raising concern about stability in the nuclear-armed country as it grapples with an economic crisis.
Police took Khan from his home in the city of Lahore on Saturday and transferred him to a jail in Attock district, near the capital Islamabad, where a court convicted him of graft charges arising from the unlawful sale of state gifts and sentenced him to three years in prison.
"I met Imran Khan who told me that 'they've put me in a C-class'," lawyer Naeem Panjutha said on Monday, referring to conditions in the jail where he said he spent just under two hours with Khan preparing paperwork for filing his appeal.
"It is a small room which has got an open washroom where he said there were flies in the daytime and insects in the night."
Khan's legal team was also appealing to authorities to secure him better conditions in jail, Panjutha told reporters in Islamabad earlier.
Political prisoners are entitled to better "B-class" facilities, including access to television, newspapers and books.
A government spokesperson and the prison authorities did not respond to repeated requests for comment on the conditions in which Khan is being held.
The graft conviction, which Khan rejects as politically motivated, likely means he will be disqualified from running in a general election due by November.
Khan's arrest was the latest in a series of blows that have weakened his political standing after he fell out with the powerful military and his party splintered.
Ever since his ouster, Khan has been campaigning for a snap election and organising protests, which led to significant violence on May 9, raising tension with the military.
Khan accuses the military and his political opponents of plotting against him to block him from the election. The military, which has ruled Pakistan for about half its history, denies that.
Prime Minister Shehbaz Sharif is expected to call this week for the dissolution of parliament paving the way for a general election by November.
The political crisis has played out alongside an economic one.
Last month, the International Monetary Fund's board approved a $3 billion bailout for Pakistan to help it tackle an acute balance of payments crisis and dire shortage of central bank reserves.
